Here is what the Library of Congress has to say about sunscreens:
Sunscreen works by combining organic and inorganic active ingredients. Inorganic ingredients like zinc oxide or titanium oxide reflect or scatter ultraviolet (UV) radiation. Organic ingredients like octyl methoxycinnamate (OMC) or oxybenzone absorb UV radiation, dissipating it as heat. Some sunscreens protect us from the two types of damaging UV radiation: UV-A and UV-B. Both UV-A and UV-B cause sunburns and damaging effects such as skin cancer.
Ultraviolet radiation is broken into three types of wavelengths:
UV-A: This is the longest wavelength and is not absorbed by the ozone. It penetrates the skin deeper than UV-B.
UV-B: Responsible for sunburns. It is partially blocked by the ozone layer.
UV-C: This is totally absorbed by the earth's atmosphere; we encounter it only from artificial radiation sources.
When purchasing sunscreen, the Sun Protection Factor or SPF measures how effectively the sunscreen formula limits skin exposure to UV-B rays that burn the skin. The higher the SPF the more protection the sunscreen will provide against UV-B rays. SPF does not measure UV-A. If you are looking for UV-A protection, the experts recommend that you purchase a product that has broad-spectrum protection.
I recommend to patients broad spectrum sunscreens with the SPF of 45. If you use SPF over 45, the rule is still the same: reapply your sunscreen every 2 hours, after each swimming, and when you sweat profusely. Avoid sun between 10 a.m. and 4 p.m., stay in shade as much as possible, and wear a hat and sunglasses! Teach your children these habits early in life, because we get the most sun exposure and sun damage before age 18 years.